THE ATXANTOIS. 



381 



cavity, and spreads itself out on the inner surface of the 

 chorion. In many Mammals the allantois becomes so largo 

 that it finally surrounds the whole embryo with its other 

 appendages, as a great covering, and extends over the whole 

 inner surface of the egg-membrane. On cutting such an 

 egg, the first thing met with is a large space filled with 



Fig. 136. — Embryo of Dog, twenty -five days old, opened on the ventral side 

 (as in Figs. 134 and 135). Chest and ventral walls have been removed: a, 

 nose-pits; b, eyes ; c, under -jaw (first gill-arch) ; d, second gill-arch ; efgh, 

 heart (e, right, /, left auricle ; g, right, h, left ventricle) ; ' i, aorta (origin of ) ; 

 kk, liver (in the middle between the two lobes is the cut yelk-vein); i, 

 stomach ; m, intestine ; n, yelk-sac ; 0, primitive kidneys : p, allantois ; 3, 

 fore -limbs ; h, hind-limbs. The crooked embryo has been stretched straight. 



